A service that we have been using since it was Grand Central is Google Voice. It is currently a free service that gives you one number for all your phones, voicemail that is easy as email, and many enhanced calling features like call blocking and screening. You can read more about it here Google Voice.
You can also sign up for the beta rollout which should happen in the next few weeks. It also can be integrated with the iPhone using a great application (soon to be released) called GV Mobile. More on that later …